1. Field of the Invention
The present invention relates to a sheet processing apparatus which functions to apply saddle stitching and folding in the middle to a plurality of sheets.
2. Description of the Related Art
There has been widely provided a sheet processing apparatus which can perform bookbinding including saddle stitching processing and mid-folding processing on a sheet bundle containing a large number of sheets that have undergone image formation by an image forming apparatus. When mid-folding processing is performed on a sheet bundle by such a sheet processing apparatus, it can happen that the fold line portion of the sheet bundle is not properly folded, so that the folded sheet bundle swells and looks rather unsatisfactory. In particular, when the number of sheets is large or in the case of thick sheets, the fold line portion may be easily opened.
To solve the above issue, in Japanese Patent Application Laid-Open No. 2003-182928, after the fold line portion has been formed through mid-folding processing, a press roller is moved in a direction orthogonal to a sheet conveyance direction while pressing the fold line portion. The pressing helps to properly fold the fold line portion and improve an outward appearance of a booklet prepared.
However, when the fold line portion of the sheet bundle that has undergone saddle stitching and mid-folding is pressed by the press roller, it can happen that, due to the pressure applied, the outermost sheet (cover) is pulled, with the result that sheet may be ripped in the vicinity of the binding portion. If, to prevent generation of such cover ripping, the pressing force of the press roller were reduced, it would be impossible to maintain the appearance quality of the fold line portion.